1. Field of the Invention
This invention is in the field of lightning protection for wind turbine blades and other objects.
2. Description of the Related Art
At present there are three primary methods proposed in the industry for protecting wind turbine blades from lightning strikes: tip receptors, surface receptors, and blade coverings.
Tip receptors are typically a cylindrical conductor that protrudes through the distal fin of the blade or a conductive end cap on the distal fin, which matches the blade profile. Some of these receptors are equipped with condensation drains. They are always connected to an earthing conductor, or earthing electrode, running laterally through the cavity of the turbine blade terminating at the blade root. Some configurations use an anchoring block, known as a receptor block, located within the blade cavity some distance beneath the tip as an electro-mechanical bonding junction for the receptor and earthing conductor. Tip receptors are sometimes proposed in conjunction with surface receptors.
Surface receptors are conductor assemblies with an exposed face aligned to the exterior surface of the blade. The body of the external facing receptor disk is recessed into the blade wall occasionally extending into the blade cavity. The exposed face is usually circular, while the shapes of the bodies vary. Methods for attachment also vary, but typically surface receptor disks are partially encased in the blade wall and fastened into a receptor block. Surface receptors are often placed on the pressure and suction sides of the blade, and there can be several of them at various points along the length of the blade. The number of surface receptors is often dependent upon the length of the blade. They are connected to a root earthing conductor either directly, through branch conductors, or through electrical bus-like junctions such as a receptor block. Blade protection designs augmented with either diverter strips or conductive blade coverings have been proposed in conjunction with these receptors as a means of channeling the lightning to the receptor.
Blade coverings are typically conductive layers that cover the surface of the blade. The layers may be continuous solids, meshes, or cured resins. They are applied in sheets, strips or patches. The conductors may be placed on the surface or just below a surface coating.
Wind turbine blades are most commonly made of glass fiber reinforced plastic (GFRP/FRP), which is more commonly known as fiberglass. Even though fiberglass is non-conductive, blades are common attachment points for lightning strikes. With the introduction of blade lightning protection systems, internal conductive components of the blade lightning protection system are competing with the receptors as the strike termination point. Lightning damage to wind turbine blades can occur in several ways: 1) an upward leader originating from an object within the blade instead of the external face of a receptor; 2) “tracking” or shift in the attachment point during the lightning conduction phase; 3) side-flash or arcing within the blade during the lightning conduction phase; and/or 4) damage or fatigue has also been reported due to streamer currents in the pre-lightning attachment phase.
Being the less conductive object, the fiberglass shell and structural reinforcement, or spar, acts as an invisible skin for the lightning protection skeleton. The conductive objects become “stressed” as the electrical field intensifies in conditions suitable for lightning. Three main factors affect the field stress around a conductive object: 1) orientation of the object with respect to the electrical field, 2) position of the object with respect to neighboring conductive objects, and 3) geometry of the conductive object. Conductive objects enter a phase known as corona when the localized electrical field stress intensifies to a sufficient threshold. The next progression from corona is streamer initiation. Streamer initiation can escalate to upward leader initiation. A conductive path is formed and electrons begin to flow once an upward leader attaches to a downward stepped leader emanating from overhead clouds. This process describes downward initiated lightning, or cloud-to-ground lightning.
Lightning damage to installed blades has led some in the industry to draw the conclusion that current practices fail to adequately deter the initiation of upward leaders from conductive elements of the receptor, or more specifically, the internally mounted lightning protection system components. Current receptors typically do not insulate the internal receptor components or connections with anything other than bonding paste and/or the resin that is used to bond the glass fiber. Recognizing the shortfall in wind turbine blade lightning protection systems, a new international standard, IEC 61400-24, “Lightning Protection for Wind Turbines,” has been ratified to address criteria for blade components and systems, including receptors.
Insulating the receptor block and cable connections by surrounding or encapsulating the conductive components in a relatively high dielectric strength medium may not be sufficient to prevent these internal components from generating upward leaders. Vestas Wind Systems A/S details such an approach in published patent applications WO 2008/101506 and WO 2007/128314 (AU AU2007247617).
It will be appreciated that further improvements in this field of endeavor would be desirable.